Nowadays different methods of the sea exploring for location of hydrocarbons deposits are widely used. These methods are application-dependent and include excitation electromagnetic field pulses acting upon the seabed, subsequent registration of the changes in the near-bottom strata electromagnetic parameters, and analysis of the obtained data for detection of existing anomalies and determination of their nature. Such exploring is realized using different research complexes of apparatus and equipment (RC), as, for instance, taught in RU 2236028, 2004; SU 1122998, 1984; SU 1798666, 1996; SU 1434385, 1988; U.S. Pat. No. 4,298,840, 1981; U.S. Pat. No. 4,617,518, 1986.
There is a well-known method of sea electrical survey on a drifting ice-floe provided by means of a research complex consisting of exciting and receiving arrays, a source of alternating-sign periodical current pulses and a data processing system (RU2069375, 1993). The essence of this method is that exciting and receiving arrays are placed upright under ice in the water layer. The pulses feeding are provided by the source of alternating-sign periodical current pulses of several dozens amperes by means of ERS 72 or another system, the data processing is provided by means of a digital electrical survey system of a TSESS type. The section profiling is realized with a fixed distance between the points of signals excitation and receiving after, wherein the excitation is provided by alternating-sign pulses. Only vertical component of the electric field is measured in this case. However, this method can't be used in measurement during a vessel movement, as it is based on excitation a water layer that is immovable in relation_to the ice-floe. Moreover, this method may not be employed in the case of depth more than 200 m.
For the vessels survey, the methods involving the analysis of data on the strata polarization under the action of electromagnetic field (the method of induced polarization, the TEM method etc.) are more universal and promising (RU 2236028, 2004; SU 1122998, 1984; SU 1798666, 1996; SU 1434385, 1988; U.S. Pat. No. 4,298,840, 1981;U.S. Pat. No. 4,617,518, 1986).
So, during the carrying out the survey according to the method of induced polarization (RU 2094829, 1993), one registers the density of the electromagnetic field of the induced polarization, representing the difference between the strength of the complete field and the strength of the field caused by the process of the induced polarization. According to the results of analysis of this parameter changes depending on time, one may judge about presence of geo-electric irregularities and their nature.
A typical RC used for the sea electrical survey includes a vessel-carrier of apparatus, provided with an echo-sounder, towed horizontal dipole with the source electrodes and receiving electrodes. The complete set of the apparatus located on the vessel includes pulse formation units for registration and analysis of the obtained data and auxiliaries, ensuring that the vessel is sited to the point of research, providing a fixation of the sea depth etc. (RU 1819354, 1990).
The disadvantage of this method and the RC used therefor is its practical unsuitability for the shelf research with the sea depth more than 150-200 m wherein the polarizing influence on the seabed strata is not measurable.
The most similar to the present invention by the technical essence and achieved effect is the complex of sea electrical survey and a RC used therefor, with a conditional name CSEM (L. MacGregor, M. Sinha/Geophysical Prospecting, 2000, 48, 1091-1106; GB 2402745, 2003), that allows to carry out the survey at depths up to 3 km.
The essence of that method (L. MacGregor, M. Sinha/Geophysical Prospecting, 2000, 48, 1091-1106) is that the electromagnetic pulses are transmitted from the horizontal dipole, submerged into water at a depth close to the sea depth at this place, and the reflected pulses are received by bottom systems previously placed on the sea bottom. The obtained data is compared with analogous data obtained in a similar region, where hydrocarbons deposits are absent, and on the base of such comparison one can draw a conclusion on the availability of hydrocarbons deposits in the region.
For obtaining the indicated data the horizontal dipole with the moment about 104 Am is towed in a region of the bottom system at a distance about 50 m from the seabed. The dipole radiates a continuous pulse signal of electromagnetic field with a frequency 0,25-4 Hz. Since the electrical resistivity of the seawater is lower that the resistivity of the seabed, such signal is quickly damped in the water, and as a result in the process of measurement at the distance more than 500 m from the radiation source the bottom system receives only signals referred to the resistivity of the seabed strata. As a result, the receivers of the bottom systems register two orthogonal components of the horizontal electric field at the distance up to 15 km from the source. Examination of the changes in amplitude and phase of the obtained signal permits to obtain information on the electric resistivity of the strata at depths up to 5-7 km.
The RC used therefor (described in GB 2402745, 2003) comprises a vessel, upon which a generator is mounted, the outboard apparatus (OA) containing a unit of rectangular pulses formation and connected with the generator by means of a connecting cable to feed the OA, a horizontal electric dipole with the length of about 100 m with the dipole moment of about 104 Am and bottom systems of different types.
Disadvantages of the CSEM technology are the possibility of obtaining limited information volume on the seabed strata, in particular, impossibility during the use of this technology to obtain the data on the strata chargeability, essentially and this reduces the prognosis accuracy.